Compression Bump - Team Camaro Tech
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post #1 of 36 (permalink) Old Sep 27th, 16, 08:27 AM Thread Starter
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Dave
 
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Compression Bump

I think I already know the answer, but on the off chance there's another way, here goes...
Are there any other tricks to bump up compression on a BBC? I have the heads off right now. I know about these, but are there any others that I'm not thinking of? I have a calculated ratio of 8.33:1 and I'd like to increase it at least a point.

1. Thinner head gaskets (already done. .020 metal)
2. Mill heads (tempted. .006 per cc?) Problem is the heads aren't worth anything and I don't want to put any more money in them.
3. Deck block. (Don't want to disassemble short block)
4. Higher compression pistons (Same as 3)

My current specs:
Bore & Stroke 4.155 x 3.76 (.030 over 402)
Heads 112cc 148 truck heads (could be 122cc... I need to cc them tonight)
Pistons - Silvolite 1445 +30 (-8)
Head gaskets - .020 x 4.37 metal

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

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post #2 of 36 (permalink) Old Sep 27th, 16, 08:59 AM
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Dave
 
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Re: Compression Bump

Longer stroke also builds more compression than a shorter stroke. In other words you will have a high compression with a 4.5 inch stroke and flat top pistons at zero deck than you will with a 3.76 inch stroke and flat top pistons at zero deck. A BBC needs to have a dome to fill the huge combustion chambers. The problem is the bigger the dome the more it interferes with flame propagation.

One solution to this problem is a second spark plug on the lee side of the dome. Shame that none of the head manufactures have though of this as Chrysler has done this with the Hemi's for years. Of course if the manufactures read the tech boards they wouldn't need to invest as much in R&D with fewer failed projects.

You can build nearly twice the power of a 396 (402) with a SBC 400 for about half the cost of a BBC, if built with modern heads and a modern roller cam. The BBC requires a large displacement in excess of 540 cubes to build comparable power if peak horsepower is your goal in life. I wouldn't put anything but rings and bearings into your rebuild and look for a 4.500" to 4.600" bore block to build instead if you want those big valve covers.

Big Dave
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post #3 of 36 (permalink) Old Sep 27th, 16, 09:09 AM Thread Starter
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Re: Compression Bump

Quote:
Originally Posted by Larger Dave View Post
Longer stroke also builds more compression than a shorter stroke. In other words you will have a high compression with a 4.5 inch stroke and flat top pistons at zero deck than you will with a 3.76 inch stroke and flat top pistons at zero deck. A BBC needs to have a dome to fill the huge combustion chambers. The problem is the bigger the dome the more it interferes with flame propagation.

One solution to this problem is a second spark plug on the lee side of the dome. Shame that none of the head manufactures have though of this as Chrysler has done this with the Hemi's for years. Of course if the manufactures read the tech boards they wouldn't need to invest as much in R&D with fewer failed projects.

You can build nearly twice the power of a 396 (402) with a SBC 400 for about half the cost of a BBC, if built with modern heads and a modern roller cam. The BBC requires a large displacement in excess of 540 cubes to build comparable power if peak horsepower is your goal in life. I wouldn't put anything but rings and bearings into your rebuild and look for a 4.500" to 4.600" bore block to build instead if you want those big valve covers.

Big Dave
Thanks Dave. I'm just trying to work with what I have for right now. Eventually I'll get some bigger cubes to play with. I'm not looking to build a race motor with this, but I'd like it to build some respectable power without putting much more money into it if possible. If it isn't, I'll live. Just trying to exhaust possibilities before I put it back together this weekend.
I'm tempted to mill the heads, but I'd need to shave off .0057" to get up to 9.5:1. I don't know if that's doable or cost effective with these heads.

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

Semper Fi!

"The galleries are full of critics. They play no ball, they fight no fights. They make no mistakes because they attempt nothing."
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post #4 of 36 (permalink) Old Sep 27th, 16, 09:31 AM
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ray
 
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Re: Compression Bump

Would probably need .057 off to change that it that much.
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post #5 of 36 (permalink) Old Sep 27th, 16, 09:47 AM
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Re: Compression Bump

Milling the heads was my first thought, the other was changing pistons and there is the thought of a small street blower. But all these thoughts get more expensive as the ideas came to me. When I thought of pistons, I recall guys using the Speed O Motive pistons. I recall using them in race cars, I know they were very cheap in material as one guy said. But then you are basically doing expensive stuff again and you said you just wanted to get by for now.

All you want at the moment is a seat of pants feel of power. Can you find a used 454 somewhere? Decking block as Dave mentioned could work but you need to tear down entirely. Did you say once you had access to smaller chamber heads? #290 head with a 105cc chamber?

Used 502?

Don
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post #6 of 36 (permalink) Old Sep 27th, 16, 10:49 AM
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Re: Compression Bump

Back in the 70-80's we used to angle mill the heads on big blocks for higher compression, and to help the entry angle for the intake ports. More was milled off the exhaust side. You had to mill the intake flange to compensate for the intake manifold. Exhaust flange angle needed to be watched/altered too.
It was a good strategy-check with your machinist.
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post #7 of 36 (permalink) Old Sep 27th, 16, 11:02 AM
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Re: Compression Bump

Yes, I did to, something .100 on the exhaust side , when I had big domes, but his bump is in the center of the piston, he has closed chamber piston.

Don
TC # 349
Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #8 of 36 (permalink) Old Sep 27th, 16, 06:56 PM
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Re: Compression Bump

Find the smallest closed chamber oval port heads you can and install the 2.19/1.88 valves, have them pocket ported while putting the valves in, or find a set already done, nobody wants the iron oval ports, should be cheaper

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post #9 of 36 (permalink) Old Sep 27th, 16, 07:23 PM
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Re: Compression Bump

Quote:
Originally Posted by 1971ls6 View Post
Find the smallest closed chamber oval port heads you can and install the 2.19/1.88 valves, have them pocket ported while putting the valves in, or find a set already done, nobody wants the iron oval ports, should be cheaper
He is struggling to maintain what compression he has, pocket porting may take material away.
Like he said, he's beating a dead horse.

Don
TC # 349
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post #10 of 36 (permalink) Old Sep 27th, 16, 08:00 PM
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Re: Compression Bump

Simple and inexpensive solution. Run it like it is and hit it with 100 -150 shot. Will work great with the Lowe comp and the risk of detonation is minimal.

Best bang for the buck.
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post #11 of 36 (permalink) Old Sep 28th, 16, 07:14 AM Thread Starter
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Re: Compression Bump

Thanks a lot guys. There's definitely a lot of options if I want to spend more money, but I don't right now. I like John's idea if I were worried about taking someone in a race or something right now, but I'm just looking for day to day power.
I talked to Holbrook Racing last night. Good guy to talk to. I didn't realize that I'd have to mill the intake also if I milled the heads that much. Makes sense. I don't really care about these heads so I don't want to ruin my intake by matching it to junk heads. It would have run ~$400 too. $250 for the heads and another $150 for the intake. Not worth it.
I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine. I could easily throw a cam in that one and add some 2.25/1.88 valves and it would be plenty of power for what I'm looking for. It's already 10.25:1.
Eventually I'll just find something with bigger cubes and build it with the right pistons and heads.

Quote:
Originally Posted by carcrazy615 View Post
Would probably need .057 off to change that it that much.
You're right. I added an extra zero...

Quote:
Originally Posted by 1971ls6 View Post
Find the smallest closed chamber oval port heads you can and install the 2.19/1.88 valves, have them pocket ported while putting the valves in, or find a set already done, nobody wants the iron oval ports, should be cheaper
I'd do that, but I haven't seen any decent heads for reasonable money in a while. If you know of any give me a holler. I won't be putting them on until this weekend. I might shoot up to see you on Sat if you'll be around.

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

Semper Fi!

"The galleries are full of critics. They play no ball, they fight no fights. They make no mistakes because they attempt nothing."
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post #12 of 36 (permalink) Old Sep 28th, 16, 09:33 AM
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Re: Compression Bump

Just so you know it's only worth roughly 4% more horsepower for every full point increase up to 10:1 and around 2% for every full point increase above 10:1. Where you're at now is too low for sure. There's really only one way around it and you know what it is......new heads. Did you check our racingjunk.com as I mentioned before?
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post #13 of 36 (permalink) Old Sep 28th, 16, 09:58 AM
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Re: Compression Bump

"I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine."


What I don't understand is that you have these 290 heads from you born with motor and you don't want to run. This motor has the peanut pistons with matching heads.

These heads don't have an expiration date on them. Sounds like you are saving the born with motor also for ??

This is why I suggest finding a 454, 502 or 572 to play with. Maybe you can find a 427 motor complete with square ports and rebuild it as you will have more compression to play with and good flowing heads. Sounds like you are willing to spend extra bucks to get around running this peanut motor.
You are spending dollars to same dimes. Just my 2

Don
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post #14 of 36 (permalink) Old Sep 28th, 16, 11:33 AM Thread Starter
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Re: Compression Bump

Quote:
Originally Posted by onefiness View Post
Just so you know it's only worth roughly 4% more horsepower for every full point increase up to 10:1 and around 2% for every full point increase above 10:1. Where you're at now is too low for sure. There's really only one way around it and you know what it is......new heads. Did you check our racingjunk.com as I mentioned before?
I'd just like to get the compression up somewhere where the cam needs it to be. Above 9:1. I did check racingjunk and go back once in a while. I'll keep looking.

Quote:
Originally Posted by zdld17 View Post
"I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine."


What I don't understand is that you have these 290 heads from you born with motor and you don't want to run. This motor has the peanut pistons with matching heads.

These heads don't have an expiration date on them. Sounds like you are saving the born with motor also for ??

This is why I suggest finding a 454, 502 or 572 to play with. Maybe you can find a 427 motor complete with square ports and rebuild it as you will have more compression to play with and good flowing heads. Sounds like you are willing to spend extra bucks to get around running this peanut motor.
You are spending dollars to same dimes. Just my 2
"Peanut pistons"? I've never heard that term before. Only peanut plugs and that's not what that motor is. Are you referring to something else?
Not too much to understand on the heads and motor. It's irreplaceable if I break it. Sure I could get something date coded correct again some day, but this is the virgin VIN #s matching motor to an L34 X66 Camaro we're talking about. 1 of 2,018 built and a lot fewer than that with 4 speeds like I have. Think about all the guys going crazy and dropping insane amounts of money to track down and get their original motors back. I have mine. I just don't want to screw with it unless I decide to go back to a bone stock trailer/garage queen some day. Until then, I don't want to be afraid to side step the clutch at 6 grand...
I always keep my eyes open for big cube motors and I'd absolutely love to find a 427, but they're big bucks for just the blocks everywhere I see now. I'm in no rush and I keep my eyes open. The wife and our 5 and a 16 year olds have the priority with my money these days.

'69 SS396 X66 L34 08D M21 BS
460ci Gen VI 9.75:1 AED 950 carb
241/249 @ .050" .625/.625 110 LSA

Semper Fi!

"The galleries are full of critics. They play no ball, they fight no fights. They make no mistakes because they attempt nothing."
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post #15 of 36 (permalink) Old Sep 28th, 16, 02:39 PM
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Re: Compression Bump

You have the perfect compression for a supercharger! Or like said above throw a 150 shot at it.
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